Technical Field
The present disclosure relates to an aluminum alloy wire rod used as a conductor of an electric wiring structure, an aluminum alloy stranded wire, a covered wire, a wire harness and a method of manufacturing an aluminum alloy wire rod.
Background Art
In the related art, a so-called wire harness has been used as an electric wiring structure for transportation vehicles such as automobiles, trains, and aircrafts, or an electric wiring structure for industrial robots. The wire harness is a member including electric wires each having a conductor made of copper or copper alloy and fitted with terminals (connectors) made of copper or copper alloy (e.g., brass). With recent rapid advancements in performances and functions of automobiles, various electrical devices and control devices installed in vehicles tend to increase in number and electric wiring structures used for these devices also tend to increase in number. On the other hand, for environmental friendliness, lightweighting of transportation vehicles is strongly desired for improving fuel efficiency of transportation vehicles such as automobiles.
As one of the measures for achieving lightweighting of transportation vehicles, there have been, for example, continuous efforts in the studies of using aluminum or aluminum alloys as a conductor of an electric wiring structure, which is more lightweight, instead of conventionally used copper or copper alloys. Since aluminum has a specific gravity of about one-third of a specific gravity of copper and has a conductivity of about two-thirds of a conductivity of copper (in a case where pure copper is a standard for 100% IACS, pure aluminum has approximately 66% IACS), an aluminum conductor wire rod needs to have a cross sectional area of approximately 1.5 times greater than that of a copper conductor wire rod to allow the same electric current as the electric current flowing through the copper conductor wire rod to flow through the pure aluminum conductor wire rod. Even an aluminum conductor wire rod having an increased cross section as described above is used, using an aluminum conductor wire rod is advantageous from the viewpoint of lightweighting, since an aluminum conductor wire rod has a mass of about half the mass of a pure copper conductor wire rod. It is to be noted that “% IACS” represents a conductivity when a resistivity 1.7241×10−8 Ωm of International Annealed Copper Standard is taken as 100% IACS.
However, a pure aluminum wire rod, typically an aluminum alloy wire rod for transmission lines (JIS (Japanese Industrial Standard) A1060 and A1070), is generally known for being poor in its tensile strength, resistance to impact, and bending fatigue characteristics. Therefore, for example, a pure aluminum wire rod cannot withstand a load abruptly applied by an operator or an industrial device while being installed to a car body, a tension at a crimp portion of a connecting portion between an electric wire and a terminal, and a bending fatigue loaded at a bending portion such as a door portion. On the other hand, when an alloyed wire rod containing various additive elements added thereto is used, an increased tensile strength and enhanced bending fatigue characteristics can be achieved, but there has been a problem that a conductivity may decrease due to a solid solution phenomenon of the additive elements into aluminum, and because of hardening, an ease of routing and handling in attaching a wire harness may decrease, which may decrease the productivity. Therefore, the additive elements are limited or selected within ranges which would not decrease the conductivity, and it is further necessary to provide the bending fatigue characteristics and the flexibility simultaneously.
For example, aluminum alloy wire rods containing Mg and Si are known as high strength aluminum alloy wire rods. A typical example of this aluminum alloy wire rod is a 6000 series aluminum alloy (Al—Mg—Si based alloy) wire rod. Generally, the strength of the 6000 series aluminum alloy wire rod can be increased by applying a solution treatment and an aging treatment. However, when manufacturing an extra fine wire such as a wire having a wire size of less than or equal to 0.5 mm using a 6000 series aluminum alloy wire rod, although a high conductivity and high bending fatigue characteristics can be achieved by applying a solution treatment and an aging treatment, a yield strength (0.2% yield strength) increases and a large force is required for plastic deformation, and thus there is a tendency that a work efficiency of installation to a car body decreases.
A conventional 6000-series aluminum alloy wire used for an electric wiring structure of a mobile body is described, for example, in Japanese Patent No. 5607853. Japanese Patent No. 5607853 is document of a patent based on a patent application filed by the present inventors on the basis of the results of the research and development performed by the present inventors, wherein average crystal grain sizes at the outer periphery and at the interior of a wire rod are defined, and while maintaining the extensibility and conductivity higher than or equivalent to those of the related art products, an appropriate yield strength and a high bending fatigue resistance are achieved simultaneously.
However, when an aluminum alloy wire rod is used at a position to which vibration from an engine portion including an engine is applied or in the vicinity of such a position, a high vibration resistance is required. On the other hand, when an aluminum alloy wire rod is used at a door portion, a bending operation is repeatedly applied to the aluminum alloy wire rod due to the opening and closing of the door, and accordingly a flexibility (flex resistance) is required. Since the bending in the door portion and the vibration of the engine portion give different strains to the aluminum wire rod, in order to use an aluminum alloy wire rod at both of these portions, the aluminum alloy wire rod is required to have characteristics capable of sufficiently withstanding at least these two types of strains, and thus further studies of the alloy composition and the alloy structure were necessary. Japanese Patent No. 5607853 is an invention in which the peripheral grain size is refined and preferentially precipitated at the periphery in order to strengthen the surface layer of a wire rod, and the temperature history until the solution formation and the production conditions of the line tension in a wire drawing step are not taken into consideration, and no control has been performed with respect to voids and an Fe-based crystallized material in the aluminum alloy wire rod.
The present disclosure is related to providing an aluminum alloy wire rod capable of achieving both a high vibration resistance property and a high bending fatigue resistance property while ensuring a high conductivity and an moderately low yield strength even when used as an extra fine wire (for example, the strand diameter is less than or equal to 0.5 mm), an aluminum alloy stranded wire, a covered wire and a wire harness, and to provide a method of manufacturing such an aluminum alloy wire rod.
The present inventors have found that, in the precipitation type Al—Mg—Si based alloys with which a high strength and a high conductivity can be obtained, which have hitherto been continuously studied, voids present in a matrix accelerate propagation of cracks generated by vibration, and the propagation of cracks causes shortening of the use-life. The present inventors have also found that due to a frictional force (drawing force) in the die during wire drawing, voids tend to be generated particularly around coarse Fe-based compound particles. In addition, it has been found that in a usual mass production process, the wire drawing is performed continuously by using 10 to 20 dies, and accordingly all the frictional forces are concentrated in the wire rod immediately before winding up. In contrast to this, it has been found that the stress loaded on the wire rod can be decreased by limiting the number of dies used near the final wire size or by arranging, between dies, a pulley to decrease a line tension. Also, if all the line tensions are decreased, the mass productivity will greatly decrease. Accordingly, a method has been found in which the line tensions only in vicinity of the final wire size, at which an effect is significant, are decreased. It has also been found that the Fe-based compound particles can be refined by increasing the casting cooling rate in order to decrease coarse Fe-based compound particles, and by shortening other heat treatment times. However, when refinement of the Fe-based compound particles is performed excessively, an effect of suppressing the coarsening of crystal grains of the alloy is lost to some extent. Accordingly, the additive components of the alloy and the manufacturing process have been studied again to find a method with which both the generation of voids and the coarsening of the crystal grains can be suppressed, and thus the present disclosure has been completed.